1. Field of the Invention
The present invention relates to a method and apparatus for measuring one or more of the hematocrit, hemoglobin concentration, mean corpuscular volume, red blood cell count, mean cellular hemoglobin, mean cellular hemoglobin concentration, and total protein concentration of a blood sample.
2. Discussion of the Background
Physicians routinely test blood parameters as part of the diagnostic process. The complete blood count (CBC) is the most common of these tests. Physicians use the results to assess the quantity and the condition of the blood's cellular components. Three of the elements of the complete blood count are used to describe the size and number of red blood cells in the sample: the hematocrit, the mean corpuscular volume, and the red blood cell count. Furthermore, four more blood properties describe the oxygen-carrying capacity of the red blood cells: the hemoglobin concentration, total protein concentration, the mean cellular hemoglobin, and the mean cellular hemoglobin concentration.                HCT—Hematocrit—(typical units: percent or unitless ratio)        HGB—Hemoglobin Concentration (typical units: g/dl)        MCV—Mean Cellular Volume (typical units: fl)        RBC—Red Blood Cell Count (typical units: cells per μl)        MCHC—Mean Cellular Hemoglobin Concentration (typical units: g/dl)        MCH—Mean Cellular Hemoglobin (typical units: pg)        TPC—Total Protein Concentration (typical units: g/dl)        
Hematocrit (HCT) is one of the most important blood parameters to be calculated by the CBC. HCT is defined as the portion of the total volume of blood occupied by red blood cells. This volume fraction may be expressed as a decimal (e.g., liter/liter) or as a percentage (e.g., liter/liter×100%). HCT measurements typically provide the same information to the physician as the hemoglobin concentration (HGB) or total protein concentration (TPC)—the oxygen carrying capacity of the blood—because under normal physiological conditions almost all of the total protein in the blood is hemoglobin and it is contained in the red blood cells.
Mean Corpuscular Volume (MCV) is the average of the red blood cell volume. The Red Blood Cell Count (RBC) is an expression of the number of red blood cells per unit volume of blood, typically, cells per microliter (μl). Mean Cellular Hemoglobin (MCH) is the average mass of hemoglobin that can be found in each red blood cell. In contrast, Mean Cellular Hemoglobin Concentration (MCHC) is the average concentration (instead of mass) of hemoglobin in red blood cells.
These blood properties, in particular HCT or HGB, can be used to diagnose anemia, acute blood loss, dehydration, and scores of other conditions. HCT or HGB can also be used to assess the oxygen carrying capability of the blood. Physicians monitor HCT and HGB both acutely and chronically and may act on changes of as little as two percent (2%) of the measured value.
These seven blood metrics are intrinsically related and dependent. For example, one relation between the above properties is that the concentration of hemoglobin in a blood cell is simply the mass of hemoglobin divided by the volume it occupies: MCHC=MCH/MCV meaning MCHC can be calculated from the other two instead of being independently measured. Among the other components of blood that are characterized in a complete blood count include white blood cells and platelets. Whole blood is defined as blood that includes red blood cells, white blood cells, platelets, and all the normal components of blood.
In the hospital environment, the blood lab routinely performs complete blood counts. Blood samples are drawn into vials and delivered to the central blood lab where an automated system performs the testing. The results are relatively accurate, but not immediately available (typically requires 10 minutes to 1 hour). Alternatively, some handheld blood parameter devices provide measurements of HCT or HGB at the point of care, but the relative inaccuracy inherent in these devices limits their diagnostic value to that of a screening test.
In the emergency medical environment, there is currently no method to measure HCT in the field. The handheld devices described above are difficult to use or are not sufficiently accurate. Patients requiring a hematocrit measurement, such as victims of trauma or disaster, must await transport to a hospital or clinic with a blood lab before this information is available. If this information were available in the field, it would improve the ability of medical personnel to triage patients and speed the delivery of appropriate medical care when the patient arrived at the hospital.
In the field, it can be difficult to assess the extent to which an injured patient has bled internally. A patient's HCT decreases with blood loss. Consequently, successive HCT measurements provide a valuable indication of the degree of blood loss. In cases where the emergency medical personnel are overwhelmed by the number of injured, a device which quickly measures the HCT of those in need of medical care would greatly improve the ability of the emergency medical personnel to focus their attention on critical cases. Thus, the public emergency medical industry and the military have a significant need for a device and method capable of measuring HCT quickly, accurately and at point-of-care.
Private practice physicians who need accurate measurements of HCT are currently limited to sending blood samples to a contract blood lab, or performing slow, imprecise manual techniques that are subject to human error such as spun hematocrit or microscopic inspection.
Four methods are currently available to measure HCT:                centrifuge,        cell count,        optical characteristics, and        electrical characteristics.        
The centrifuge method is the most basic measurement technique. These centrifuges are not portable. To measure HCT, a blood sample is drawn and spun in a centrifuge (e.g. READACRIT®) for a fixed duration (typically five to thirty minutes, depending on protocol). The spin separates the blood sample into three layers. The top layer is the plasmas made up primarily of water and dissolved solids. The next layer is the thin buffy coat, made up of white blood cells, plasma proteins, and platelets. The bottom layer contains closely packed red blood cells. A technician reads the volume fraction directly using a scale. Spun hematocrit accuracy can be affected by user error in reading the scale, plasma entrapped in the red blood cell column, and distortion of red blood cell size. Typically, the resulting accuracy of a spun hematocrit performed to protocol is 2 to 5% of the measured value. The accuracies in this document are reported as the 95% confidence intervals around the mean.
Cell counting is the most direct of the measurement techniques. The blood sample is diluted to a known ratio and individual cells are counted either manually or automatically. Manual cell counting techniques are tedious and proper preparation of the sample depends on the skill of the operator. Automated cell counters (e.g. COULTER@ GEN S™ System) typically offer 1-minute sample turnaround, claim accuracies to 2.0-3.5% of the measured value, and reduce tedium and operator dependence. As a practical matter, the turnaround time at the point of care is typically 30 minutes to 12 hours, because blood samples must be transported from the patient to the centrally located lab, processed, and the results must be reported back to the point of care. Furthermore, automated systems are typically expensive and are not portable.
The optical measurement technique is relatively new. Devices employing this technique measure the amount of light transmitted through, or reflected from, flowing blood. These devices (e.g. 3M™ CDI™ System 500) are designed for use during cardiac surgery, require a blood circuit, and are not portable.
HemoCue®, is an example of a handheld device that photometrically measures the blood hemoglobin concentration. Such portable photometric devices have a 1-minute cycle time, but the accuracy is typically around 3%. A portable device with greater accuracy would be valuable because physicians make decisions based on changes as small as 1-2% of the reading.
Electrical conductivity is currently used to measure a variety of blood parameters, including hematocrit. The i-STAT® system, for example, measures the conductivity of a blood sample, corrects for ion concentrations, assumes normal white blood cell and protein levels and then calculates and reports hematocrit. While instruments that use electrical conductivity are portable, the accuracy of a typical conductivity-based hematocrit reading is ±6%, which substantially reduces the clinical value.
In the field of blood ultrasonics, much investigation has focused on analyzing ultrasonic backscatter in devices that measure blood flow velocity using the Doppler effect. These studies are useful for understanding the interaction between ultrasound and blood. Also, many researchers have explored the ultrasonic characteristics of blood for the purpose of better understanding how these characteristics enable or interfere with imaging and sonography devices.
Schneditz et al built a sound-speed sensor and evaluated it as a method for measuring total protein concentration. The device is intended to track fluid shifts in a patients blood as they are on a hemodialysis machine. These fluid shifts would manifest themselves as a change in total protein concentration. Schneditz et al investigated the correlation between total protein concentration and speed of sound in order to detect these fluid shifts. Schneditz et al implemented a speed of sound measurement by measuring time of flight along a single direct path. A disadvantage of the Schneditz et al device is that it only works with continuously circulating blood from the patient and back into the patient. The blood must be continuously flowing in order to avoid settling of the blood cells from the plasma, which would cause inaccurate readings. Another disadvantage is that it requires a large volume (60 mL) of blood circulating through tubing from a thermostatted 500 mL bath, and it requires calibration with reference fluids whose speed of sound was known accurately.
The Schneditz et al device was implemented on porcine blood with the white blood cells artificially removed (along with any other blood components in the white blood cell layer). The absence of white blood cells and the physical differences between porcine blood and human blood may significantly alter the ultrasonic response of the blood and therefore the Schneditz et al correlations and methods may not apply to whole or human blood.
Conventional methods for measuring temperature, including thermostat-controlled baths are cumbersome and impractical. Other methods, such as directly contacting the blood with a temperature probe, lead to cleaning and contamination complications. None of the above apparatus or methods solve the problems of speed, accuracy, and portability in hematocrit or hemoglobin concentration measurement. Only the present invention achieves all three goals simultaneously.